In a slipforming process, the concrete mix is extruded with the help of auger feeders into a mold or through nozzles, whereby the casting machine is propelled along the casting bed by the reaction forces imposed on the auger feeders. The ready-cast product remains on the casting bed. Conventionally, the entire length of the long casting bed is configured to be cast according to the needs of that individual piece of concrete slab having the most stringent specifications in the casting run. Then, this specific slab dictates the pretensioning steel tendons, their pretension forces and the concrete mix grade to be used in the casting run. Resultingly, an extra high amount of reinforcing steel and concrete mix of an unnecessarily high grade are wasted in many of the other slabs to be cut from the long slab cast on the bed. Such extra costs can be reduced, e.g., by way of using a concrete mix grade adjusted to meet the specifications of each individual slab.
Now the present invention makes it possible to cast concrete products of different quality specifications in a single slip-form casting run by virtue of providing the casting machine with a compartmentalized feed hopper and then controlling the concrete mix feed from the compartments according to the progress of the casting run along the length of the casting bed.
A two-compartment slip-form casting technique known in the art is the so-called slideformer method, wherein two or three layers of concrete are cast sequentially in order to obtain a desired end product. All the concrete layers are fed from different feed hoppers and, generally, all the hoppers are filled with the same concrete mix grade. In exceptional cases, a different concrete grade such as suitable for making exposed aggregate products, for instance, is cast into the bottom layer of the slab or to the surface layer thereof. In the prior-art method, each one of the cast concrete mix layers is compacted separately.
In the method and apparatus described in FI Pat. Appl. No. 991,165 for fabrication of fiber-reinforced concrete products by extruder casting technique, a slip-form casting machine is disclosed having its feed hopper partitioned into at least two compartments and equipped with a control gate adapted to cover the feed opening provided at the bottom of either one of the two feed compartments. In this system, one compartment of the feed hopper contains standard-grade concrete mix, while the other compartment is filled with fiber-reinforced mix. In this fashion, the control gate of the feed hopper can be operated to adjust the feed ratio of the different concrete mixes so as to obtain a desired kind of end product quality. The primary function of this apparatus is to provide a nonhomogeneous distribution of fiber reinforcement in the volume of the end product.
It must be noted that the patent application described above concerns only the feed ratio of the fiber-reinforced mix to the standard-grade concrete mix. The application does not propose the use of plural types of other possible concrete mix grades.